Isotropic Nd2Fe14B-type melt spun materials have been used for making bonded magnets for many years. Although Nd2Fe14B-type bonded magnets are found in many cutting edge applications, their market size is still much smaller than that of magnets made from anisotropic sintered ferrites (or ceramic ferrites). One of the means for diversifying and enhancing the applications of Nd2Fe14B-type bonded magnets and increasing their market is to expand into the traditional ferrite segments by replacing anisotropic sintered ferrite magnets with isotropic bonded Nd2Fe14B-type magnets.
Direct replacement of anisotropic sintered ferrite magnets with isotropic bonded Nd2Fe14B-type bonded magnets would offer at least three advantages: (1) cost saving in manufacturing, (2) higher performance of isotropic bonded Nd2Fe14B magnets, and (3) more versatile magnetizing patterns of the bonded magnets, which allow for advanced applications. Isotropic bonded Nd2Fe14B type magnets do not require grain aligning or high temperature sintering as required for sintered ferrites, so the processing and manufacturing costs can be drastically reduced. The near net shape production of isotropic bonded Nd2Fe14B bonded magnets also represents a cost savings advantage when compared to the slicing, grinding, and machining required for anisotropic sintered ferrites. The higher Br values (typically 5 to 6 kG for bonded NdFeB magnets, as compared to 3.5 to 4.5 kG for anisotropic sintered ferrites) and (BH)max values (typically 5 to 8 MGOe for isotropic bonded NdFeB magnets, as compared to 3 to 4.5 MGOe for anisotropic ferrites) of isotropic Nd2Fe14B-type bonded magnets also allows a more energy efficient usage of magnets in a given device when compared to that of anisotropic sintered ferrites. Finally, the isotropic nature of Nd2Fe14B-type bonded magnets enables more flexible magnetizing patterns for exploring potential new applications.
To enable direct replacements of anisotropic sintered ferrites, however, the isotropic bonded magnets should exhibit certain specific characteristics. For example, the Nd2Fe14B materials should be capable of being produced in large quantity to meet the economic scale of production for lowering costs. Thus, the materials must be highly quenchable using current melt spinning or jet casting technologies without additional capital investments to enable high throughput production. Also, the magnetic properties, e.g., the Br, Hci, and (BH)max values, of the Nd2Fe14B materials should be readily adjustable to meet the versatile application demands. Therefore, the alloy composition should allow adjustable elements to independently control the Br, Hci, and/or quenchability. In addition, the isotropic Nd2Fe14B-type bonded magnets should exhibit comparable thermal stability when compared to that of anisotropic sintered ferrite over similar operating temperature ranges. For example, the isotropic bonded magnets should exhibit comparable Br and Hci characteristics compared to that of anisotropic sintered ferrites at 80 to 100° C. and low flux aging losses.
Conventional Nd2Fe14Btype melt spun isotropic powders exhibit typical Br and Hci values of around 8.5–8.9 kG and 9 to 11 kOe, respectively, which make this type of powders usually suitable for anisotropic sintered ferrite replacements. The higher Br values could saturate the magnetic circuit and choke the devices, thus preventing the realization of the benefit of the high values. To solve this problem, bonded magnet manufacturers have usually used a non-magnetic powder, such as Cu or Al, to dilute the concentration of magnetic powder and to bring the Br values to the desired levels. However, this represents an additional step in magnet manufacturing process and thus adds costs to the finished magnets.
The high Hci values, especially those higher than 10 kOe, of conventional Nd2Fe14B type bonded magnets also present a common problem for magnetization. As most anisotropic sintered ferrites exhibit Hci values of less than 4.5 kOe, a magnetizing field with peak magnitude of 8 kOe is sufficient to fully magnetize the magnets in devices. However, this magnetizing field is insufficient to fully magnetize certain conventional Nd2Fe14B type isotropic bonded magnets to reasonable levels. Without being fully magnetized, the advantages of higher Br or Hci values of conventional isotropic Nd2Fe14B bonded magnet can not be fully realized. To overcome the magnetizing issues, bonded magnet manufacturers have used powders having low Hci values to enable a full magnetization using the magnetizing circuit currently available at their facilities. This approach, however, does not take full advantage of the high Hci value potential.
Many improvements of melt spinning technology have also been documented to control the microstructure of Nd2Fe14B-type materials in an attempt to obtain materials of higher magnetic performance. However, many of the attempted efforts have dealt only with general processing improvements without focusing on specific materials and/or applications. For example, U.S. Pat. No. 5,022,939 to Yajima et al. Claims that use of refractory metals provides a permanent magnet material exhibiting high coercive force, high energy product, improved magnetization, high corrosion resistance, and stable performance. The patent claims that the addition of the M element controls the grain growth and maintains the coercive force through high temperatures for a long time. Refractory metal additions, however, often form refractory metal-borides and may decrease the Br value of the magnetic materials obtained, unless average grain size and refractory metal-borides can be carefully controlled and uniformly dispersed throughout the materials to enable exchange coupling to occur. Further, the inclusion of refractory metals in alloy composition, as disclosed in the Yajima patent may actually narrow the optimal wheel speed window for achieving high performance powders.
U.S. Pat. No. 4,765,848 to Mohri et al. claims that the incorporation of La and/or Ce in rare earth based melt spun materials reduces material cost. However, the alleged reduction in cost is achieved by sacrificing magnetic performance. Moreover, this patent does not disclose ways in which the quenchability of melt spun precursors may be improved. U.S. Pat. Nos. 4,402,770 and 4,409,043 to Koon disclose the use of La for producing melt spun R—Fe—B precursors. However, these patents do not disclose how to use La to control the magnetic properties, namely the Br and Hci values, to desired levels.
U.S. Pat. No. 6,478,891 to Arai claims that the use of 0.02 to 1.5 at % of Al in an alloy with nominal composition of Rx(Fe1−yCoy)100−x−z−wBzAlw, where 7.1≦x≦9.0, 0≦y≦0.3, 4.6≦z≦6.8 and 0.02≦w≦1.5, improves the performance of materials composed of hard and soft magnetic phases. The patent, however, does not disclose the various impact of Al addition, e.g., on the phase structure and on the wetting behavior during melt spinning or jet casting processes.
Arai et al., IEEE Trans. on Magn., 38:2964–2966 (2002), reports that a grooved wheel with ceramic coating can improve the magnetic properties of melt spun materials. This claimed improvement, however, involves a modification of current jet casting equipment and process, and therefore is unsuitable for using existing manufacture facilities. Moreover, the approach only addresses melt spinning processes using relatively high wheel speeds. In a production situation, however, high wheel speed is usually undesirable because it makes the process more difficult to control and increases machine wear.
Therefore, there is still a need for isotropic Nd—Fe—B type magnetic materials with relatively high Br and Hci values and exhibiting good corrosion resistance and thermal stability. There is also a need for such materials to have good quenchability, e.g., during rapid solidification processes, such that they are suitable for replacement of anisotropic sintered ferrites in many applications.